A storable array is an IO-mutable array which stores its
contents in a contiguous memory block living in the C
heap. Elements are stored according to the class Storable.
You can obtain the pointer to the array contents to manipulate
elements from languages like C.

It is similar to IOUArray but slower.
Its advantage is that it's compatible with C.

The pointer to the array contents is obtained by withStorableArray.
The idea is similar to ForeignPtr (used internally here).
The pointer should be used only during execution of the IO action
retured by the function passed as argument to withStorableArray.